In recent years, instant messaging (IM) has attracted attention as communication means on IP networks such as the Internet. IM is a service that combines presence services to permit referencing of the states (presences) of partners such as whether partners are connected to the network and message exchange services that perform character-based conversations in real-time such as chats and progress has been made in standardizing such services (See RFC2778 and RFC2779).
Further, the application of such IM services to mobile phones has also been proposed and Wireless Village, the Mobile Instant Messaging and Presence Services (IMPS) Initiative, which is standards body for IM service-related technology using mobile phones has been established, and common specifications that are also used for the exchange of presence information and messages between Internet-based services instead of between mobile devices have been decided on.
Further, in recent years, there has been a progressive focus on SIP (Session Initiation Protocol)-compliant IP telephone systems, SIP is a signaling protocol for multimedia conferences and VoIP (Voice over Internet Protocol). These SIP-compliant IP telephone systems are also capable of providing presence services (RFC3261).
An SIP-compliant IP telephone system is called an ‘SIP phone system’ hereinbelow. Further, VoIP clients, which are terminals of an SIP phone system, include multifunctional telephones, PCs and PDAs (soft phones) and so forth comprising SIP-phone client functions, and are referred to collectively as ‘SIP phones’ or ‘SIP phone terminals’.